Coping in groups of domestic horses – Review from a social and neurobiological perspective

Many sport horses live solitary, with no physical contact to other horses allowed for years, for many different reasons including limiting risk of injuries caused by other horses. Other horses are (semi-) permanently housed in large groups, because their owners perceive this is “natural” and thus good for their horses’ welfare. Indeed, sometimes this leads to increased aggression and individuals who cannot cope with social stress and limited space. However, usually humans determine the social composition of these groups of geldings and mares, possibly leading to other interactions as known from natural equine societies. In this review paper the underlying neurobiological mechanisms of social relationships and social dynamics in modern horses were assessed. Ultimately, it can help to define hypotheses to determine strategies what could be most optimal non-voluntary social compositions of domestic groups and adjust introduction techniques so the individuals can cope optimally, i.e. to show the lowest ethological and physiological stress indicators over both short and long period of time. Based on neurobiological evidence it is shown that indispensable behaviours like exploration and grooming are displayed because of their rewarding properties. Endogenous opioids play a pivotal role in rewarding the individual and motivate it to perform these behaviours, irrespective of the short-term biological success of such indispensable behaviours. Equine allogrooming and play in domestic groups have been addressed to assess whether they fulfilled the four ethological need criteria: (i) performed by all individuals; (ii) self-rewarding; (iii) have a rebound effect and (iv) in absence of a proper substrate (i.e. social partner) chronic stress is induced. Indeed it was assessed whether equine allogrooming and play are ethological needs. Not only in domestic non-voluntarily composed groups but it is evidenced why it is also an ethological need for individually housed sport horses. Summarising: social relationships and interventions especially in relation to preferred partners, have rewarding properties in the brain and seem therefore important for the main coping mechanisms of domestic horses in large groups and most likely also for feral horses.